The amplitude characteristics of radar echoes depend on radar and geometric parameters as well as the physical characteristics of the objects from which the echoes emanate. In particular, the echo characteristics also depend on polarization, both that used for transmission and that used for reception.
The radar cross section .sigma. is a useful quantity for determining echo strength. This quantity is defined as EQU .sigma.=4.pi.R.sup.2 (p.sup.r /p.sup.t)
where the power density at the target is p.sup.t, the power density at the radar is p.sup.r and the distance from the target is R.
The terminology .sigma..sub.AB is used to denote radar cross section (RCS) for transmit polarization A and receive polarization B. For example, let the symbol x denote a linear polarization in any direction and let y denote a polarization perpendicular to x. In that case, .sigma..sub.xy denotes RCS for transmitting any linear polarization and receiving a linear polarization perpendicular to that transmitted. Because of reciprocity, .sigma..sub.xy =.sigma..sub.yx. It is noted here that received signal power, other factors being equal, is proportional to .sigma..
The reflecting properties of an object depend on various factors including surface shape, size, material, orientation, as well as the frequency and polarization of the radar signals. Object shape and orientation may cause waves to be reradiated with a polarization different from that incident on the object, i.e., .sigma..sub.AB .noteq.0. Such shapes are said to produce depolarization. Most large objects that have smooth surfaces, either metallic or dielectric, do not cause depolarization. Therefore, there are many targets of interest that cause less depolarization than is caused by most land an sea surfaces.
The concept of using the depolarization effects of clutter to discriminate between the clutter and a target has been discussed in several patents. Apparatus for differentiating between target echoes and clutter echoes based on transmitting a linearly polarized wave, and receiving that polarization and one perpendicular to that polarization has been described in U.S. Pat. No. 2,508,571 issued to E. L. Hudspeth; U.S. Pat. No. 3,566,403 issued to E. G. Hills; U.S. Pat. No. 3,849,780 issued to F. J. Dynan; and U.S. Pat. No. 3,918,059 issued to D. J. Adrian. Apparatus for differentiating between target echoes and clutter echoes on transmitting a circular polarized wave, and receiving both left-and right-circularly polarized echoes have been described in U.S. Pat. No. 3,403,397 issued to R. C. Harrington and G. R. Latham IV; U.S. Pat. No. 3,614,787 issued to G. E. Hart; and U.S. Pat. No. 3,755,810 issued to G. R. Latham and W. T. Allen.
As reported in the book Radar Reflectivity of Land and Sea, D. C. Heath and Company, Lexington, Mass., 1975, p. 338, by M. W. Long, there are troublesome target-like sea echoes, called sea spikes, that are only slightly depolarized; i.e., the echoes are almost entirely of one polarization. Therefore, the dual polarized radar systems described above are ineffective for suppressing such clutter.
There are two patents by T. Nirasawa and H. Ota that use various parameters, called discriminants herein, for reducing the effects of sea clutter. U.S. Pat. No. 3,893,117 contains claims using two pulse lengths and two beamwidths, and U.S. Pat. No. 4,008,472 contains claims using two polarizations and two carrier wavelengths. The embodiments provide means for comparing the video amplitudes for one value, called operative herein, of a discriminant with another value of the same discriminant. Nirasawa and Ota provide means for gating-off the receiver outputs when the ratio of video amplitudes is beyond a present value and for gating-off the receiver output when that ratio is between two preset values. A limitation of this prior art is that the ratios for clutter can have almost any value and can vary at rats as fast as 200 Hz or more for sea clutter. This means that clutter will often be present in each radar resolution cell used. It should be noted that there are fundamental differences between the gating means used by Nirasawa and Ota and those used in this disclosure for improved suppression of clutter, antenna sidelobes, and interference from external sources. Also, although there are one-antenna embodiments that use pulse length as the discriminant, all other embodiments included by Nirasawa and Ota require the use of two antennas. Further, the embodiments of Nirasawa and Ota limit the signal processing means to those using either the video output of one receiver, the other receiver, or the sum of the outputs of the two receivers.
It is noted that the prior art of Nirasaw and Ota provides for using wavelength as a discriminant without indicating a need to maintain a closeness of frequency so that two simultaneously received video signals will be equal or nearly equal for a target. Further, they use separate transmitters without means for maintaining a frequency difference and two antennas are used for carrier wavelengths .lambda..sub.1 and .lambda..sub.2. According to Nirasawa and Ota the antennas are identical in construction with each other except that they are designed for the carriers .lambda..sub.1 and .lambda..sub.2 respectively.